This invention relates to apparatus for continuously producing photovoltaic devices on a web of magnetic substrate material by depositing successive amorphous-silicon alloy semiconductor layers in each of at least two adjacent deposition chambers. The composition of each amorphous layer is dependent upon the particular process gases introduced into each of the deposition chambers. The gases introduced into the first deposition chamber are carefully controlled and isolated from the gases introduced into the adjacent deposition chamber. More particularly, the deposition chambers are operatively connected by a relatively narrow gas gate passageway (1) through which the web of substrate material passes; and (2) adapted to isolate the process gases introduced into the first chamber from the process gases introduced into the adjacent deposition chamber. As disclosed in U.S. patent application Ser. No. 372,937, filed Apr. 29, 1982, and entitled "Magnetic Gas Gate," it has been determined that despite the relatively small size of the gas gate passageway, dopant gases introduced into one chamber back diffuse into the adjacent chamber, thereby contaminating the layer deposited in said adjacent chamber. The "Magnetic Gas Gate" application disclosed apparatus (namely magnets positioned above the passageway opening for urging the magnetic substrate upwardly) by which the height of the passageway opening in the gas gate could be reduced. The reduction in the height of the passageway opening correspondingly reduced the back diffusion of dopant gases for the same flow rates, thereby decreasing the contamination of the layer deposited in the intrinsic deposition chamber. However, it has been determined that when the web of substrate material is urged by the magnets against the upper wall of the gas gate passageway, the passageway is divided by the web of substrate material into a relatively wide lower slit and a relatively narrow upper slit. (For purposes of the instant application the term "slit" shall be defined as the spacing, however irregular it may be, between the upper surface of the substrate and the upper wall of the gas gate passageway.) The process gases, being inherently viscous (and especially viscous at the elevated deposition temperatures), are unable to travel through the narrow upper slit with sufficient velocity to prevent back diffusion of process gases from the dopant deposition chamber into the adjacent intrinsic deposition chamber. It is to the end of decreasing the back diffusion of process gases through the narrow upper slit, between the unlayered surface of the web of substrate material and the wall of the passageway opening, that the present invention is directed.
Recently, considerable efforts have been made to develop processes for depositing amorphous semiconductor alloys, each of which can encompass relatively large areas, and which can be doped to form p-type and n-type materials for the production of p-i-n-type devices which are, in operation, substantially equivalent to their crystalline counterparts. For many years such work with amorphous silicon or germanium films was substantially unproductive because of the presence therein of microvoids and dangling bonds which produce a high density of localized states in the energy gap. Initially, the reduction of the localized states was accomplished by glow discharge deposition of amorphous silicon films wherein silane (SiH.sub.4) gas is passed through a reaction tube where the gas is decomposed by a radio frequency (r.f.) glow discharge and deposited on a substrate at a substrate temperature of about 500-600 degrees K. (227-327 degrees C.). The material so deposited on the substrate is an intrinsic amorphous material consisting of silicon and hydrogen. To produce a doped amorphous material, phosphine gas (PH.sub.3), for n-type conduction, or diborane (B.sub.2 H.sub.6) gas, for p-type conduction is premixed with the silane gas and passed through the glow discharge reaction tube under the same operating conditions. The material so deposited includes supposedly substitutional phosphorus or boron dopants and is shown to be extrinsic and of n or p conduction type. The hydrogen in the silane was found to combine, at an optimum temperature, with many of the dangling bonds of the silicon during the glow discharge deposition to reduce the density of the localized states in the energy gap.
It is now possible to prepare greatly improved amorphous silicon alloys, that have significantly reduced concentrations of localized states in the energy gaps thereof, while providing high quality electronic properties by glow discharge. This technique is fully described in U.S. Pat. No. 4,226,898, Amorphous Semiconductors Equivalent to Crystalline Semiconductors, Stanford R. Ovshinsky and Arun Madan which issued Oct. 7, 1980 and by vapor deposition as fully described in U.S. Pat. No. 4,217,374, Sanford R. Ovshinsky and Masatsugu Izu, which issued on Aug. 12, 1980, under the same title. As disclosed in these patents, fluorine introduced into the amorphous silicon semiconductor operates to substantially reduce the density of the localized states therein and facilitates the addition of other alloying materials, such as germanium.
Activated fluorine readily diffuses into, and bonds to, amorphous silicon in a matrix body to substantially decrease the density of localized defect states therein. This is because the small size of the fluorine atoms enables them to be readily introduced into an amorphous silicon matrix. The fluorine bonds to the dangling bonds of the silicon and forms a partially ionic stable bond with flexible bonding angles, which results in a more stable and more efficient compensation or alteration than could be formed by hydrogen, or other compensating or altering agents which were previously employed. Fluorine is considered to be a more efficient compensating or altering element than hydrogen when employed alone or with hydrogen, because of its exceedingly small size, high reactivity, specificity in chemical bonding, and having highest electronegativity.
Compensation may be achieved with fluorine, alone or in combination with hydrogen, upon the addition of such element(s) in very small quantities (e.g., fractions of one atomic percent). However, the amounts of fluorine and hydrogen most desirably used are much greater than such small percentages, permitting the elements to form a silicon-hydrogen-fluorine alloy. Thus, alloying amounts of fluorine and hydrogen may, for example, be used in a range of 0.1 to 5 percent or greater. The alloy thus formed has a lower density of defect states in the energy gap than can be achieved by the mere neutralization of dangling bonds and similar defect states. In particular, it appears that use of larger amounts of fluorine participates substantially in effecting a new structural configuration of an amorphous silicon-containing material and facilitates the addition of other alloying materials, such as germanium. Fluorine, in addition to the aforementioned characteristics, is an organizer of local structure in the silicon-containing alloy through inductive and ionic effects. Fluorine, also influences the bonding of hydrogen by acting to decrease the density of the defect states which hydrogen normally contributes. The ionic role that fluorine plays in such an alloy is an important factor in terms of the nearest neighbor relationships.
The concept of utilizing multiple cells, to enhance photovoltaic device efficiency, was discussed at least as early as 1955 by E. D. Jackson, U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures therein discussed utilized p-n junction crystalline semiconductor devices. Essentially the concept is directed to utilizing different band gap devices to more efficiently collect various portions of the solar spectrum and to increase open circuit voltage (Voc.). The tandem cell device has two or more cells with the light directed serially through each cell, with a large band gap material followed by a smaller band gap material to absorb the light passed through the first cell or layer. By substantially matching the generated currents from each cell, the overall open circuit voltage is increased without substantially decreasing the short circuit current.
Many publications on crystalline stacked cells following Jackson have been reported and, more recently, several articles dealing with Si-H materials in stacked cells have been published. Marfaing proposed utilizing silane deposited amorphous Si-Ge alloys in stacked cells, but did not report the feasibility of doing so. (Y. Marfaing, Proc. 2nd European) Communities Photovoltaic Solar Energy Conf., Berlin, West Germany, p. 287, (1979).
Hamakawa et al., reported the feasibility of utilizing Si-H in a configuration which will be defined herein as a cascade type multiple cell. The cascade cell is hereinafter referred to as a multiple cell without a separation or insulating layer therebetween. Each of the cells was made of an Si-H material of the same band gap in a p-i-n junction configuration. Matching of the short circuit current (J.sub.sc) was attempted by increasing the thickness of the cells in the serial light path. As expected, the overall device Voc. increased and was proportional to the number of cells.
Due to the beneficial properties attained by the introduction of fluorine, amorphous alloys used to produce cascade type multiple cells now incorporate fluorine to reduce the density of localized states without impairing the electronic properties of the material. Further band gap adjusting element(s), such as germanium and carbon, can be activated and are added in vapor deposition, sputtering or glow discharge processes. The band gap is adjusted as required for specific device applications by introducing the necessary amounts of one or more of the adjusting elements into the deposited alloy cells in at least the photocurrent generation region thereof. Since the band gap adjusting element(s) has been tailored into the cells without adding substantial deleterious states, because of the influence of fluorine, the cell alloy mantains high electronic qualitites and photoconductivity when the adjusting element(s) are added to tailor the device wavelength characteristics for a specific photoresponse application. The addition of hydrogen, either with fluorine or after deposition, can further enhance the fluorine compensated or altered alloy. The post deposition incorporation of hydrogen is advantageous when it is desired to utilize the higher deposition substrate temperatures allowed by fluorine.
It is of obvious commercial importance to be able to mass produce photovoltaic devices. Unlike crystalline silicon which is limited to batch processing for the manufacture of solar cells, amorphous silicion alloys can be deposited in multiple layers over large area substrates to form solar cells in a high volume, continuous processing system. Continuous processing systems of this kind are disclosed, for example, in pending patent applications: Ser. No. 151,301, filed May 19, 1980 for A Method of Making P-Doped Silicon Films and Devices Made Therefrom; Ser. No. 244,386, filed Mar. 16, 1981 for Continuous Systems For Depositing Amorphous Semiconductor Material; Ser. No. 240,493, filed Mar. 16, 1981 for Continuous Amorphous Solar Cell Production System; Ser. No. 306,146, filed Sept. 28, 1981 for Multiple Chamber Deposition and Isolation System and Method; and Ser. No. 359,825, filed Mar. 19, 1982 for Method And Apparatus For Continuously Producing Tandem Amorphous Photovoltaic Cells. As disclosed in these applications, a substrate may be continuously advanced through a succession of deposition chambers, wherein each chamber is dedicated to the deposition of a specific material. In making a solar cell of p-i-n-type configuration, the first chamber is dedicated for depositing a p-type amorphous silicon alloy, the second chamber is dedicated for depositing an intrinsic amorphous silicon alloy, and the third chamber is dedicated for depositing an n-type amorphous silicon alloy. Since each deposited alloy, and especially the intrinsic alloy must be of high purity, the deposition environment in the intrinsic deposition chamber is isolated from the doping constituents within the other chambers to prevent the back diffusion of doping constituents into the intrinsic chamber. In the previously mentioned patent applications, wherein the systems are primarily concerned with the production of photovoltaic cells, isolation between the chambers is accomplished by gas gates through which unidirectional gas flow is established and through which an inert gas may be "swept" about the web of substrate material. The gas gate disclosed in previously mentioned patent application Ser. No. 372,937 contemplated the creation of a plurality of magnetic fields adapted to urge the magnetic web of substrate material against a wall of the passageway opening so that the height of the passageway opening could be reduced. The reduced width of the opening correspondingly decreased the amount of process gas which back diffused from the dopant deposition chambers to the intrinsic deposition chamber without correspondingly increasing the risk that amorphous layers, deposited on the substrate, would contact a passageway wall.
While the magnetic gas gate of patent application Ser. No. 372,937 reduced the height of the gas gate passageway opening, and thereby reduced the levels of contamination previously encountered, it simultaneously divided the passageway opening into a relatively wide lower slit and a relatively narrow upper slit. The velocity of the inert sweep gases and residual process gases traveling through the wide lower slit is sufficiently great to substantially prevent the back diffusion of process gases from the dopant deposition chamber to the intrinsic chamber. However, due to the fact that the sweep gases employed in the deposition apparatus are viscous, which viscosity becomes more pronounced at the elevated temperatures required by the apparatus to deposit amorphous semiconductor layers onto the substrate, the drag on the sweep gases along the passageway wall and the unlayered surface of the substrate which define the relatively narrow upper slit results in a relatively low velocity flow therethrough. Accordingly, the process gases from the dopant chamber are able to back diffuse into the intrinsic chamber through the narrow upper slit.
Referring now to the drawing of FIG. 6, the parabola referenced by the alphabetical character A indicates the velocity profile of the gases flowing from the intrinsic chamber to the dopant chamber through the relatively large lower passageway slit 84, while the alphabetical character B indicates the velocity profile of the gases flowing from the intrinsic chamber to the dopant chamber through the relatively narrow upper passageway slit 82. By comparing the two velocity profiles, it is readily apparent that the velocity of the gases flowing through the lower, relatively large passageway slit, is far greater than the velocity through the upper, relatively narrow passageway slit. Further, since the narrow upper slit is caused by random warping and canoeing of the thin substrate material, the degree of contamination fluctuates with time, resulting in nonuniform semiconductor layers.
At this point is it necessary to discuss pressure differential relative to the back diffusion of gases between the adjacent deposition chambers operatively connected by the gas gate. FIG. 7 is a graph of the number of atoms of a gas per second (dn/dt) flowing through a narrow opening as a function of the size of that opening (in this case "a" indicates the gas gate passageway opening), assuming a constant pressure differential is maintained on both ends of the opening. As the size of "a" is increased, the volume of gases flowing therethrough in order to maintain the constant pressure differential must correspondingly increase. Therefore, the velocity must correspondingly increase. This represents a desirable characteristic because the greater the velocity of process gases flowing from the intrinsic deposition chamber to the dopant deposition chamber, the more difficult it becomes for dopant gases to back diffuse from the dopant deposition chamber to the intrinsic chamber. The functional dependency of back diffusion, dn/dt, relative to the size of the gas gate passageway opening "a" is represented by the equation (a) (e.sup.- a.sup.2). That functional dependency, as evidenced by the amount of back diffusion, reaches a maximum when "a" is about 200 microns or about 10 mils. This point is indicated by the alphabetical character C on the graph of FIG. 7. It is therefore essential that both, the size of the slits above, as well as below, the web of substrate material be greater than the 200 micron level at which maximum back diffusion occurs. Obviously, the size of the slits should be substantially greater than 200 microns so that back diffusion is minimized. There is no problem in creating a sufficiently large opening below the web of substrate material since said substrate is magnetically urged toward the upper wall of the passageway opening. The focus of the present invention is decreasing back diffusion in the narrow opening above the web of substrate material, the size of which has been found in prior art devices to approach the 200 micron "danger point."
The problem of back diffusion is solved in the present invention by providing a plurality of elongated grooves (extending the entire, approximately eight inch, length of the passageway opening) from the dopant deposition chamber to the adjacent intrinsic deposition chamber in the wall of the passageway opening above the web of substrate material. In this manner, a plurality of spaced, relatively high flow channels are provided in the space between the unlayered surface of the web of substrate material and the upper wall of the passageway opening. Because the channels are relatively high, the sweep gases and residual process gases are adapted to unidirectionally flow therethrough at substantial velocities despite the drag incurred as said gases contact the passageway wall and the substrate surface. Although relatively narrow slits still exist between adjacent high velocity flow channels established by the elongated grooves, it is much more probable for molecules of dopant process gases to enter the high velocity channels during their traverse of the eight inch long passageway separating the dopant chamber from the intrinsic chamber, than to have those molecules remain in the narrow slit between the high velocity flow channels for that eight inch long trip. By substantially reducing the amount of back diffusion from the dopant deposition chamber to the intrinsic deposition chamber, the production of a more efficient photovoltaic device is accomplished.
Although the foregoing discussion dealt with a single dopant deposition chamber and an adjacent intrinsic deposition chamber, it should be apparent that other deposition chambers may be operatively connected by the gas gates of the present invention. For example, a p-type deposition chamber may be connected on one side of the intrinsic deposition chamber and an n-type deposition chamber may be connected on the other side of the intrinsic deposition chamber so as to produce a p-i-n-type semiconductor device. Alternatively, a plurality of these triads of deposition chambers could be interconnected to produce a plurality of p-i-n-type cells. Similarly, a chamber in which the transparent conductive oxide layer (discussed hereinafter) is added atop the uppermost amorphous semiconductor alloy layer may be operatively connected by the instant gas gates to the final deposition chamber. Since it would be undesirable to have constituents introduced into the transparent conductive oxide (TCO) chamber back diffused into the dopant deposition chamber, the grooved gas gate of the present invention would be employed between the TCO chamber and the final dopant deposition chamber. For that matter, the grooved gas gate of the present invention is applicable between all chambers of the continuous production apparatus so as to produce amorphous photovoltaic devices of high quality.
The many objects and advantages of the present invention will become clear from the drawings, the detailed description of the invention and the claims which follow.